1. Field of the Invention
The present invention relates to a magnetic recording medium that is excellent in terms of output characteristics and durability.
2. Description of Related Art
As recording/reproducing methods are shifting from analog methods to digital methods, there is a demand for an improvement in the recording density of coated-type magnetic recording media in which a magnetic layer containing a magnetic powder and a binding agent is formed on a non-magnetic substrate. This demand increases every year in particular for high-density digital video tapes, computer backup tapes, and the like.
In accordance with such an improvement in the recording density, the recording wavelength is becoming shorter, and the particle size of the magnetic powder has been made smaller every year in order to respond to this short-wavelength recording. Currently, a ferromagnetic hexagonal ferrite powder having an average particle size of approximately 20 nm is realized, and magnetic recording media using this magnetic powder have been implemented (JP 2015-91747A, for example).
In order to further improve the recording density of magnetic recording media using the ferromagnetic hexagonal ferrite powder, the particle size of the ferromagnetic hexagonal ferrite powder has to be made even smaller. However, if the ferromagnetic hexagonal ferrite powder is made finer, the particle volume of the magnetic powder is reduced, which will result in a problem that the media is likely to be affected by heat fluctuation. Accordingly, heat fluctuation has to be suppressed using a magnetic substance that has high coercivity and large anisotropy energy even when the particle size is made smaller.
In light of such a situation, recently, ε-Fe2O3 has been studied as a new magnetic material for magnetic recording media, and an iron oxide nano magnetic particle powder constituted by an ε-Fe2O3 single phase having ferromagnetic characteristics even at an average particle size of 15 nm or less, and preferably 10 nm or less has been proposed (JP 2014-224027A, for example). Furthermore, magnetic recording media using ε-Fe2O3 as a magnetic powder have been proposed as well (JP 2015-82329A and JP 2014-149886A, for example).
Commonly known Fe2O3 crystal structures are constituted by a gamma (γ) phase or an alpha (α) phase, whereas ε-Fe2O3 is a crystal structure that is present therebetween, and exhibits magnetic anisotropy that is based on crystal anisotropy, and thus ε-Fe2O3 is characterized by exhibiting high coercivity even at a particle size that is as small as 10 nm or less.
However, it can be seen from a differential curve obtained by differentiating a hysteresis curve of ε-Fe2O3 that a peak appears in a magnetic field range of +500 oersted [Oe] or more and also in a magnetic field range of less than +500 oersted [Oe]. The reason for this seems to be that, since an ε-Fe2O3 magnetic powder contains not only high-coercivity components but also a magnetic powder constituted by components having different coercivities, in particular, low-coercivity components, and thus a peak appears also in a magnetic field range of less than +500 oersted [Oe]. If such a low-coercivity component is present in a magnetic layer of a magnetic recording medium, a magnetic signal cannot be recorded even if recording of the magnetic signal with a magnetic head is attempted, and thus this material is not preferable as a magnetic material to be used for magnetic recording media for high-density recording.
The present invention was made in order to solve the above-described problems in conventional techniques, and provides a magnetic recording medium that can achieve both high output characteristics and high durability, by setting, to a specific range, a ratio between a local maximum of the largest peak and a local maximum of the second largest peak in a specific range in a differential curve obtained by differentiating a hysteresis curve in a thickness direction of a magnetic layer.